German lager yeast appears to have originated on beech trees in southern Argentina. But how did it get to Europe 600 years ago?

How did lager beer come to be? After pondering the question for decades, scientists have found that an elusive species of yeast isolated in the forests of Argentina was key to the invention of the crisp-tasting German beer 600 years ago.

It took a five-year search around the world before a scientific team discovered, identified and named the organism, a species of wild yeast called Saccharomyces eubayanus that lives on beech trees.

"We knew it had to be out there somewhere," said Chris Todd Hittinger, an evolutionary geneticist at the University of Wisconsin-Madison and a coauthor of the report published Monday in the Proceedings of the National Academies of Sciences.

Their best bet is that centuries ago, S. eubayanus somehow found its way to Europe and hybridized with the domestic yeast used to brew ale, creating an organism that can ferment at the lower temperatures used to make lager.

Geneticists have known since the 1980s that the yeast brewers use to make lager, S. pastorianus, was a hybrid of two yeast species: S. cerevisiae — used to make ales, wine and bread — and some other, unidentified organism.

Searching through collections of wild yeasts from Europe, researchers — including Hittinger and his collaborators — tried to identify lager's missing link but again and again were stumped. "There were a few candidates, but none fit particularly well," Hittinger said.

So he and his colleagues began "sampling more systematically," collecting soil and bark, sap and abnormal growths called galls from trees on five continents.

Team member Diego Libkind of the Institute for Biodiversity and Environment Research in Bariloche, Argentina, found S. eubayanus in galls on southern beech trees in Patagonia. The galls were particularly rich in sugar, which yeast like to colonize and consume.

Patagonian natives used to make a fermented beverage from the galls — a definite clue that the scientists were on the right track, Hittinger said.

When the team brought the yeast to a lab at the University of Colorado and analyzed its genome, they discovered that it was 99.5% identical to the non-ale portion of the S. pastorianus genome, suggesting it was indeed lager yeast's long-lost ancestor.

"The DNA evidence is strong," said Gavin Sherlock, a geneticist at Stanford University who has studied lager yeast but was not involved in this study.

But Sherlock wondered how S. eubayanus could have traveled the nearly 8,000 miles from Argentina to Germany.

"We all know that in 1492, Columbus sailed the ocean blue," he said. "Lager was invented in the 1400s. It's not really clear how that progenitor would have gotten from South America to Europe."

Scientists may yet find colonies of the yeast in Europe, he said. Another possibility is that lager yeast originated a bit later than previously thought, added Barbara Dunn, a senior research scientist who works in Sherlock's lab.

"It certainly could have existed somewhere else," Hittinger acknowledged. "Just because somebody hasn't found it doesn't mean it doesn't exist."

The beech forests where the team found S. eubayanus are cool, with an average year-round temperature of 43 to 46 degrees Fahrenheit, Hittinger said.

Genes that permit the yeast to thrive in such a chilly environment probably provided S. pastorianus' ability to ferment at relatively low temperatures — conditions not too terribly different from those prevalent in the Bavarian cellars where monks created the golden brew in the 15th century, Hittinger said.

The researchers compared the DNA of the wild Patagonian yeast with that of lager yeast used in breweries to see what changes had evolved over the years. They found changes in genes that regulate sugar and sulfite metabolism, processes that contribute to the fermentation and preservation of beer.

Scientists could exploit such knowledge to improve biofuels, Hittinger said.

And, of course, tinkering with yeast genes might make wine or beer taste better too, said Hittinger, who is "a lager man" himself. Coauthor Mark Johnston, a molecular biologist at the University of Colorado School of Medicine, prefers ales.

"Even though we both stand by our original preferences, we both have a new appreciation for where lager came from and the complexity of the processes that made it what it is today," Hittinger said.